This invention relates to .alpha.-4-isocyanatophenoxy) -.omega.-(4-isocyanato-3-methylphenoxy)-alkanes and mixtures thereof with the corresponding diisocyanates free from methyl substituents and the corresponding symmetrically di-methyl-substituted diisocyanates, to a process for producing these diisocyanates and diisocyanate mixtures and to their use as synthesis component(s) in the production of polyurethane plastics.
The properties of a polyurethane plastic, particularly polyurethane elastomer, are largely determined by the nature of the polyisocyanate used in the production of the plastic. Thus, particularly high-quality polyurethane elastomers are obtained, for example, where 1, 5-diisocyanatonaphthalene is used as the diisocyanate component. In particular, cast elastomers based on this diisocyanate are distinguished by excellent mechanical properties (See for example Becker, Braun, Kunststoff-Handbuch, Vol. 7, 2nd Edition (1983), Carl-Hanser-Verlag). However, 1,5-diisocyanatonaphthalene is attended by the disadvantages that the raw material required for its production, namely naphthalene, is only available in limited quantities. In addition, the nitration of naphthalene leads inevitably to an isomer mixture of nitronaphthalenes from which 1, 5-dinitronaphthalene has to be isolated. Purification by distillation of the 1,5-diisocyanatonaphthalene obtained from the dinitro compound by hydrogenation and subsequent phosgenation of the resulting diamine also involves difficulties because it tends to sublime. The net result of this is that 1,5-diisocyanatonaphthalene is expensive to produce.
The processing of 1,5-diisocyanatonaphthalene is also difficult in many cases because its melting point and its vapor pressure are relatively high. These properties often make it impossible for 1,5-diisocyanatonaphthalene to be readily reacted in the form of a melt. Technically elaborate processing methods and protective measures are thus necessary to avoid chemical and industrial hygiene problems.
Accordingly, there has been no shortage of attempts to find an equivalent replacement for 1,5-diisocyanatonaphthalene as diisocyanate component in the production of high-quality polyurethane plastics.
DE-OS 3,138,421 and DE-OS 3,138,422, for example describe the production of polyurethane elastomers using 4, 4'-diisocyanato-1, 2-diphenylethane as the diisocyanate component. Although plastics showing favorable mechanical properties can be obtained with this diisocyanate, the production of 4, 4'-diisocyanato1, 2-diphenylethane is complicated and expensive and, hitherto, has always been difficult to achieve on an industrial scale.
Numerous attempts have also been made to use the comparatively inexpensive 4, 4'-diisocyanatodiphenylmethane instead of 1, 5-diisocyanatonaphthalene for the production of high-quality polyurethane elastomers. However, until now, all attempts to produce polyurethane elastomers based on this diisocyanate which correspond in their mechanical and thermal properties to polyurethane elastomers based on 1,5-diisocyanatonaphthalene have always ended in failure.
.alpha., .omega.-bis-(4-isocyanatophenoxy)-ethane,-n-butane and-n-hexane are described in DE-OS 3,442,689 as synthesis components for polyurethane plastics. Although the use of these diisocyanates in polyurethane elastomers does give products having outstanding properties, difficulties are sometimes encountered in the processing of these isocyanates. These diisocyanates do give the desired reaction products containing highly crystalline rigid segments but they are highly reactive. Consequently, when these diisocyanates are reacted with polyols, solids can precipitate spontaneously from the liquid reaction mixtures. Such precipitation affects the industrial application of reactions of the type in question, particularly for the production of prepolymers.
Accordingly, there is an interest in diisocyanates having the same basic structure, but reduced reactivity. A first step in this direction can be seen in the diisocyanates according to DE-OS 3,525,606 (EP-A-0,183,115). The .alpha.,.omega.-bis-(4-isocyanato-3-methylphenoxy)-alkanes described in this publication do not present any difficulties by precipitation of solids in their reaction with polyols and the polyurethane plastics obtained also show a range of outstanding properties. However, the disadvantage of these diisocyanates is that their reactivity is unexpectedly reduced severely by the two methyl substituents. Consequently, the reaction of these isocyanates with polyols generally requires the addition of catalyst. This can be unfavorable, particularly in the production of prepolymers.